If the large main wheels of a wheelchair are in a vertical plane (i.e., set at a 0.degree. camber angle) the track width is minimized. This makes it easier for the chair to pass through narrow doorways. If, on the other hand, the wheels are cambered outward at the base by some angle, the increased track width improves lateral stability, which allows for sharper turns. Although prior art wheelchairs have been made with adjustable camber angles, resetting the camber angle of a chair is at best a laborious operation requiring considerable mechanical aptitude, a selection of hand tools, and a safe place to keep small semi-custom hardware items. Most prior art chairs are made with a single camber angle that is not adjustable.
The prior art of making a wheelchair with an adjustable main wheel camber angle comprises at least the following:
A widely used receiving socket for a stub axle is designed to be bolted to a wheelchair frame with four bolts. Shims (e.g., washers placed around two of the four bolts) are used to tilt the socket about a horizontal axis. PA1 Friedrich, in U.S. Pat. No. 4,768,797, teaches an axle-receiving socket tiltable to one of a predetermined number of angles. Friedrich provides a plurality of vertically disposed sets of mounting holes on the side of a socket assembly. The holes are aligned so each set provides a different camber angle, and so that the mounting point of the wheel system is translated outward as the camber angle increases, which ensures that the top of the wheel, where the wheelchair user grasps the push rim, stays in the same position relative to the user's body. Friedrich's mounting system provides only for relatively small (e.g., three degrees of arc) changes in camber angle. PA1 McWehty, in U.S. Pat. No. 5,060,962, teaches an improvement to the receiving socket mentioned above. McWehty provides a receiving socket assembly comprising an angled cambering sleeve member. PA1 Robertson et al, in U.S. Pat. No. 5,131,672, teach a chair with a large uncambered bore in its frame. Pairs of plugs, each with its own internal cambered axle-receiving socket, are plugged into the uncambered bore to set the wheel camber angle. Robertson et al also provide a useful review of the variety of camber angles commonly employed and of wheelchair sports for which various camber angles are commonly used. PA1 A threaded stub axle is screwed into a socket on the frame. This arrangement has proven unpopular with people who have misplaced a wrench and have been unable to get the chair into a car until after the wrench was found or replaced. PA1 A hollow stub axle with a laterally protruding spring-biased ball is inserted into a socket on the frame (see, for example, FIG. 3 of U.S. Pat. No. 5,060,962)--i.e., the stub axle acts as a single-acting ball-lock pin. The exterior surface of the hollow stub axle must be lubricated. This surface, which is exposed when the wheel is demounted, stains car upholstery. PA1 Bergman, in U.S. Pat. No. 4,362,311, teaches a wheelchair in which stub axles extending from the hubs of main wheels are received in brackets having cooperating bifurcated legs. PA1 Agrillo, in U.S. Pat. No. 4,758,013, teaches a wheelchair in which a collection of tubular members surrounding a stub axle are received in bifurcated slots extending from the wheelchair's frame.
The art of demountably attaching a wheel to a wheelchair is also relevant to the present invention, particularly as it relates to light, rigid frame chairs used by wheelchair athletes. These chairs are designed with readily demountable wheels so that when the occupant of the chair transfers to an automobile, the frame and the wheels are separated before being loaded into the car. Two methods of wheel attachment for these chairs are in common use: